Methods for tuning an adaptive impedance matching network with a look-up table

ABSTRACT

Methods for generating a look-up table relating a plurality of complex reflection coefficients to a plurality of matched states for a tunable matching network. Typical steps include measuring a plurality of complex reflection coefficients resulting from a plurality of impedance loads while the tunable matching network is in a predetermined state, determining a plurality of matched states for the plurality of impedance loads, with a matched state determined for each of the plurality of impedance loads and providing the determined matched states as a look-up table. A further step is interpolating the measured complex reflection coefficients and the determined matching states into a set of complex reflection coefficients with predetermined step sizes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/762,607, filed Feb. 8, 2013, which is a continuation of U.S. patentapplication Ser. No. 13/408,624 filed Feb. 29, 2012 (now U.S. Pat. No.8,395,459), which is a continuation of U.S. patent application Ser. No.13/297,951 filed Nov. 16, 2011 (now U.S. Pat. No. 8,421,548), which is acontinuation of U.S. patent application Ser. No. 12/236,662 filed Sep.24, 2008 (now U.S. Pat. No. 8,072,285), the disclosures of which areincorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to methods for tuning animpedance matching network, and more specifically to methods for tuningan impedance matching network with a look-up table.

BACKGROUND OF THE INVENTION

Electronic communication products which transmit and receive radiofrequency (RF) signals need to match the impedances between the internalcircuitry, such as an RF power amplifier, and the antenna for optimumperformance of the product. The optimum mode may also depend upon themodulation scheme, the frequency of transmission and otherconsiderations.

Electronically tunable filters are frequently used to compensate for anyimpedance mismatch between the circuitry and the antenna. Such tunablefilters utilize various tuning elements, such as tunable capacitors,varactors, micro-electronic mechanical systems (MEMS), and dopedsemiconductor materials.

Adaptive impedance matching modules (AIMMs) sense continually senseimpedance mismatch and retune the impedance for any changed conditions.AIMMs may take up to 20 iterative steps to converge on the bestimpedance match, such as by using gradient search methods.

There is a need for an adaptive impedance matching network which canrapidly tune to the best or optimum matching impedance in fewer stepsand in less time.

SUMMARY OF THE INVENTION

The present invention is directed to methods for generating and using alook-up table relating a plurality of complex reflection coefficients toa plurality of matched states for a tunable matching network. In oneembodiment, typical steps of the methods include measuring a pluralityof complex reflection coefficients resulting from a plurality ofimpedance loads while the tunable matching network is in a predeterminedstate, determining a plurality of matched states for the plurality ofimpedance loads, with a matched state determined for each of theplurality of impedance loads, and providing the determined matchedstates in a look-up table. These steps may be reiterated for differentfrequency bands, different frequencies or for different use cases.

In other embodiments, the methods may include additional steps such asinterpolating the measured complex reflection coefficients and thedetermined matching states into a set of complex reflection coefficientswith predetermined step sizes, selecting the predetermined state used inmeasuring the complex reflection coefficients to minimize tolerancevariations based upon temperature coefficient, component tolerance ortolerance over time, using the determined matched states in the look-uptable to tune the tunable matching network, tuning the tunable matchingnetwork to adaptively match the impedance of an antenna, controlling thetunable impedance elements with digital to analog converters.

The tunable matching network plurality of tunable impedance elements,may include a which may be, for example, ferroelectric capacitors,voltage variable capacitors, MEMS, tunable inductors or networksthereof.

Yet another embodiment includes methods for generating a look-up tablerelating a set of measured parameters to a set of matched states for atunable matching network, including the steps of measuring a set ofparameters associated with a plurality of impedance loads while thetunable matching network is in a predetermined state, determining amatched state for each of the plurality of impedance loads, andproviding the determined matched states as a look-up table. The set ofmeasured parameters may be selected from the group consisting of complexreflection coefficients, current drain, incident power, reflected power,control setting of the tunable matching network, temperature, inputpower level, reliability considerations and linearity considerations.Preferably, a sufficient number of matched states are provided such thata transition from one matched state to an adjacent matched stateprovides a smooth transition.

A further embodiment may include methods of adaptively tuning a tunableimpedance matching network with a lookup table, the lookup tablecontaining a plurality of complex reflection coefficients correspondingto a plurality of impedance mismatches between an input terminal and anoutput terminal of the tunable impedance matching network, including thesteps of determining the impedance mismatch between the input and outputterminals of the tunable impedance matching network, determining theclosest impedance mismatch in the lookup table, and using the complexreflection coefficients in the lookup table which correspond to thedetermined closest impedance mismatch to tune the tunable impedancematching network. Further steps may include controlling theferroelectric capacitors with digital to analog converters, and tuningthe tunable impedance matching network to adaptively match the impedanceof an antenna.

Another embodiment may include methods of adaptively tuning a tunableimpedance matching network with a lookup table, the lookup tablecontaining sets of parameters corresponding to matched states for thetunable impedance matching network, including the steps of selecting aparameter from the group consisting of complex reflection coefficients,current drain, incident power, reflected power, control setting of thetunable matching network, temperature, input power level, reliabilityconsiderations and linearity considerations, determining the closestmatch to the selected parameter in the lookup table, and using theclosest match to the selected parameter in the lookup table to tune thetunable impedance matching network. The parameter may also be selectedto minimize tolerance variations based upon temperature coefficient,component tolerance or tolerance over time.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with its objects and the advantages thereof, maybest be understood by reference to the following description taken inconjunction with the accompanying drawings, in which like referencenumerals identify like elements in the figures, and in which:

FIG. 1 is a schematic diagram of an impedance matching circuit forincluding variable capacitors for impedance optimization;

FIG. 2 is a table of exemplary definitions of independent variables andscaling factors, related to the methods of the present invention;

FIG. 3 is a block diagram of a typical characterization set-up for themethods of the present invention;

FIG. 4 is a sample look-up table which correlates complex impedanceloads with measured complex reflection coefficients and determinedsettings of a plurality of digital to analog converters;

FIG. 5 is a table illustrating how an address pointer corresponds to theindependent variables and the adaptive impedance matching networksettings; and

FIG. 6 is another sample look-up table with interpolated settings for aplurality of digital to analog converters resulting from interpolationof measured complex reflection coefficients and determined settingswhich are found in the table shown in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

It will be understood that the present invention may be embodied inother specific forms without departing from the spirit thereof. Thepresent examples and embodiments, therefore, are to be considered in allrespects as illustrative and not restrictive, and the invention is notto be limited to the details presented herein.

FIG. 1 illustrates an impedance matching circuit, generally designated100, for impedance optimization between a source of radiofrequency (RF)power, such as an RF amplifier 102, and an RF load, such as an antenna104. An adaptive impedance matching network 106 is coupled between RFamplifier 102 and antenna 104. In the example of FIG. 1, the adaptiveimpedance matching network 106 is an adaptive impedance matching module(AIMM). AIMM 106 dynamically adjusts its internal impedance matchingcircuit to minimize reflected power to achieve a near-optimal impedancematch. For example, a common application for AIMM 106 is to correctantenna-RF amplifier impedance mismatch that often occurs with handheldradios and body-worn antennas. Similarly, handheld communicationsdevices, such as cellular telephones encounter impedance mismatch whenthe communication device is held close to the user's head or torso. Yetanother application for the AIMM 106 is in sensor networks where theantenna of the sensor can be detuned by proximity effects of the ground,foliage or debris. In general, AIMM 106 may be used in any applicationwhere forward power needs to be maximized and reverse power needs to beminimized.

AIMM 106 may be a multi-chip module comprising a tunable impedancenetwork, which contains one or more tunable ferroelectric capacitors108-109. Preferably, the tunable ferroelectric capacitors 108-109 areParatek's ParaTune™ family of passive tunable integrated circuits(PTICs), which are commercially available from Paratek Microwave, Inc.of Columbia, Md. These PTICs 108-109 utilize a Parascan® tunabledielectric material which is further described in U.S. Pat. Nos.7,107,033 and 6,514,895, which are assigned to the same assignee as thepresent invention and which are incorporated herein by reference intheir entirety. These PTICs 108-109 overcome the power limitationscommon to other tunable technologies such as varactor diodes and MEMSand can handle in excess of 2 watts of RF power with extremely lowinter-modulation distortion.

The adaptive impedance matching module or AIMM 106 in FIG. 1 has aninput terminal 110 for receiving an RF signal from RF amplifier 102. Aforward and reverse power detector 112 provides a signal on line 114 toa rectifier 116 which is representative of the forward power from RFamplifier 102. Rectifier 116 provides a rectified value of the forwardRF power to an analog to digital converter (ADC) 118, which in turnprovides a digital representation of the forward power to amicroprocessor 120. In a similar fashion, the power detector 112provides a signal on line 115 to a rectifier 117 which is representativeof the reverse or reflected power from antenna 104. Rectifier 117provides a rectified value of the reverse RF power to an ADC 119, whichin turn provides a digital representation of the reverse power to amicroprocessor 120.

Based upon the values of the determined forward and reverse powerlevels, and in accordance with one aspect of the present invention,microprocessor 120 uses a lookup table, such as table 400 in FIG. 4 ortable 600 in FIG. 6, which may be resident in memory 124 to determinebias adjustments to be made to the PTICs 108-109 for a first step ofretuning the adaptive impedance of the AIMM 106. To this end,microprocessor 124 supplies digital signals to digital to analogconverters (DACs) 122-123 which control the analog bias signals to PTICs108-109. Microprocessor 120 may continue with additional steps ofretuning the PTICs, as needed, to provide a near-optimum impedancematching between RF amplifier 102 and antenna 104.

FIG. 2 illustrates a table, generally designated 200, which is anexample of how independent variables 202 may be defined and organized.For example, the independent variables 202 may include frequencyinformation 204, reflection coefficient magnitude 205 and reflectioncoefficient phase 206. Furthermore, each of these independent variablesmay have a symbol 208, a normalized independent variable 210, anormalized symbol 212 and a number of points 214.

As shown in the example of FIG. 2, the frequency information 204 may beassociated with a symbol Fq and a normalized symbol Fq, and may consistof 12 values including one value for each transmit frequency band andone value for each receive frequency band. The frequency variable forthe frequency information 204 may also be accorded 12 points. Similarly,the reflection coefficient magnitude 205 has a symbol mag_S11, anormalized symbol N_mag_S11, and a normalized independent variabledetermined as mag_S 11 times 8 and then rounded to the nearest integer.The reflection coefficient magnitude 205 is accorded 6 points. Lastly,the reflection coefficient phase 206 has a symbol ph_S11, a normalizedsymbol Nyh_S11, and a normalized independent variable determined asdivided by 45 and then rounded to the nearest integer. The reflectioncoefficient phase 106 is accorded 8 points.

The purpose of a look-up table, such as look-up table 400 in FIG. 4 orlook-up table 600 in FIG. 6 is to make it possible for an adaptiveimpedance matching network, such as adaptive impedance matching module106 in FIG. I, to take a large first step in tuning. A large firsttuning step reduces the time to convergence of final settings, reducespower consumption and decreases traffic on the communication bus.

In accordance with one embodiment of the present invention, the look-uptable 400 may contain pairs (or sets) of digital-to-analog (DAC)settings 402 that are to be put into the high voltage applicationspecific integrated circuit (HV-ASIC) for controlling tunable impedancemodule 106. elements in the adaptive impedance matching. The pairs (orsets) of DAC settings 402 can be identified by an index which iscorrelated to independent variables 202, for example, frequency 204,magnitude S11 205 and phase S11 206. The index would be related to theposition in the table 400 of the desired information. Hence, the indexinformation would not need to be stored in the table. The index could bethe sum of three independent variables, such as independent variables202.

In this example, the address pointer, such as address pointer 602 inFIG. 6, may be calculated as pointer=6*8*Fq+8*N_ma9_(—)811+N-ph_s11. Thetable then has 432 rows (=12*6*8). Each has 2 or 3 bytes of information,one for each tunable element setting. The total memory usage for a twotunable element adaptive impedance network is 1152 bytes. The totalmemory usage for an adaptive impedance matching network with 3 tunableelements is 1728 bytes in this example.

A worst case analysis may typically be required to determine how manydifferent phases and magnitudes are sufficient or insufficient. A keyconsideration is how accurate the first step needs to be and how muchthe tolerances may degrade the accuracy. Also, the magnitude and phaseof 811 do not have to be scaled linearly. They could be scalednon-linearly to give better accuracy to areas of the Smith chart thatare common.

The operation of the adaptive impedance matching module 106 with thelook-up table 400 will now be considered. When the adaptive impedancematching module 106 is initially turned-on, it may be programmed to aninitial or default state. From the initial state, adaptive impedancematching module 106 will measure the magnitude and phase of thereflection coefficient 205 and 206, lookup the DAC setting 402 in table400 that corresponds to that reflection coefficient, and take a largefirst tuning step. The first tuning step can be improved byinterpolating between table entries. The first step may be broken intoseveral steps if the modulation accuracy or time mask specifications donot allow for a full step. The default setting may have the followingcharacteristics: a) all tunable elements may be set to the samevoltages, and b) the voltage that the tunable elements are set to wouldcorrespond to the voltage at which the tolerances have minimal impact,for example: a temperature coefficient that is 0 ppm/C. Many tolerancesmay degrade the accuracy of the first step. Using the 0 ppm/Ctemperature coefficient will reduce the contribution of temperature tothe tolerance stack-up. The default setting could also be chosen tocorrespond to the expected load impedance, such as the load-pull system306, if known.

The magnitude, phase and band information will then be used to calculatethe index of the look-up table that represents the best first guess forthe first tuning step. Thereafter, adaptive impedance matching network304 may switch to its standard adaptive search algorithm. Once optimaltuning is achieved, adaptive impedance matching network 304 can switchto a tracking mode, in which step sizes are smaller and/or less often.

Consideration will now be given to generating the look-up table, such astables 400 and 600. These tables may be generated during the designphase after the topology and the tuner values are fixed. A block diagramof the characterization test set-up 300 is shown in FIG. 3. It consistsof a signal generator 302 coupled via a conductor 303 to the adaptiveimpedance matching network 304, which in turn, is coupled via aconductor 305 to a load-pull system 306. This test set-up 300 simulatesan end use application of the adaptive impedance matching network 304,in which signal generator 302 may be an RF power amplifier, for example,in a portable communication device such as a cellular telephone,personal digital assistant, or the like. In such an end use application,the load-pull system 306 may be the RF antenna for the portablecommunication device.

The procedure to generate the table 400 begins with the adaptiveimpedance matching network 304 in a pause mode, and the tunable elementin the adaptive impedance matching network 304 set to the defaultsetting. This may be the bias voltage at which the capacitivetemperature coefficient is 0 ppm/C. Note that different settings may bepreferable if it results in improved tolerance performance. For example,if there is knowledge of the expected load, the default setting couldalso be set based on the expected load, such as presented by the loadpull system 306.

The signal generator is preferably set to the middle of the frequencyrange being characterized. As the design is better understood, thedesigner may choose to use a different frequency that better representsthe center of performance. If frequency information is available, alook-up table could be generated for each frequency, as the availabilityof memory allows or permits.

The load-pull system 306 is run through a fine mesh of the magnitudesand phases. The magnitude and phase of 811 is recorded as measured byadaptive impedance matching network 304 for each load. Alternatively,other parameters could be recorded for use in the table. Theseparameters may include (but are not limited to) complex reflectioncoefficient, current drain, incident power, reflected power, reliabilitymetrics, linearity metrics, and the like.

The adaptive impedance matching module 106 is set in the run mode. Thatis the adaptive impedance matching module 106 is permitted to optimizethe match as it would in an end use application. The optimization may bepart of the module 106 or apart from it. If the optimization is apartfrom the module 106, it may be referred to as a tunable impedancematching network. If the optimization is part of the network it may bereferred to as an adaptive impedance matching network. An adaptiveimpedance matching module 106 always includes a tunable impedancematching network.

As the load-pull system 306 runs through a fine mesh of the magnitudes404 and phases 405, the DAC settings 402 that adaptive impedancematching network 304 settles on for each load 306 are recorded, as bystorage in available memory. At this point, all of the information isavailable for a look-up table, such as look-up table 400 in FIG. 4.

Using the two data sets 304 and 305, generate contours correlating the811 measurements to the preferred DAC settings 302. The contours arethen used to interpolate to find the preferred or interpolated DACsettings 404 in table 400 (FIG. 4) for each 811 that is to berepresented in the look-up table 400.

At this point, the look-up table would be like that of table 600 in FIG.6. This is an extremely memory efficient table implementation becauseonly the output interpolated DAC settings 604 need to be stored. Anaddress pointer 602 will retrieve the applicable interpolated DACsettings 604. The interpolated DAC settings 604 are preferably organizedsuch that no searching is required, thereby saving processing time.

It will be appreciated by those skilled in the art that the above stepsof generating the look-up table, will be performed by a microprocessoror the like. For example, the above steps may be performed by amicroprocessor in the product for which the antenna matching isoccurring, such as in a cellular telephone, PDA, or the like.Alternately, the microprocessor may be provided in the adaptiveimpedance matching module 106.

While particular embodiments of the invention have been shown anddescribed, it will be obvious to those skilled in the art that changesand modifications may be made therein without departing from theinvention in its broader aspects.

What is claimed is:
 1. An apparatus, comprising: a tunable impedancematching network including a tunable reactive element, the tunableimpedance matching network being coupled with an antenna of acommunication device and positioned between the antenna and a poweramplifier of the communication device; a controller coupled with thetunable impedance matching network; and a detector coupled with thecontroller and positioned between the tunable impedance matching networkand the power amplifier, wherein the controller obtains parameters viathe detector for forward and reverse signals at a port of the tunableimpedance matching network, wherein the controller adjusts the tunablereactive element according to the parameters to perform impedancematching by locating an impedance mismatch entry in a lookup tableaccessible to the controller that approximates an impedance mismatch. 2.The apparatus of claim 1, wherein the lookup table comprises a pluralityof complex reflection coefficients predetermined by measuring a signalat the port of the tunable impedance matching network for a plurality ofimpedance loads of the antenna.
 3. The apparatus of claim 2, wherein thelookup table comprises a plurality of matched states of the tunableimpedance matching network for each of the plurality of impedance loads.4. The apparatus of claim 1, wherein the reactive element comprises avariable capacitor.
 5. The apparatus of claim 4, wherein the variablecapacitor comprises a voltage-controlled variable ferroelectriccapacitor.
 6. The apparatus of claim 5, further comprising digital toanalog converters, wherein the controller adjusts the voltage-controlledvariable ferroelectric capacitor utilizing the digital to analogconverters.
 7. The apparatus of claim 1, wherein the controller tunesthe tunable impedance matching network using one or more complexreflection coefficients to adaptively match an impedance of the antenna.8. The apparatus of claim 1, wherein the tunable reactive elementcomprises a micro-electro-mechanical systems (MEMS) device.
 9. Theapparatus of claim 1, wherein the tunable reactive element comprises asemiconductor device.
 10. The apparatus of claim 1, wherein theparameters are obtained via the detector while the tunable impedancematching network is in a predetermined state.
 11. The apparatus of claim1, wherein the communication device is a mobile phone.
 12. Acommunication device, comprising: an antenna; a tunable impedancematching network coupled with the antenna, wherein the tunable impedancematching network includes a tunable reactive element; a detector coupledwith the tunable impedance matching network; a memory that storesinstructions and a look-up table; and a processor coupled to the memory,the detector and the tunable impedance matching network, whereinresponsive to executing the instructions, the processor performsoperations comprising: obtaining one or more parameters via the detectorfor forward and reverse power associated with the tunable impedancematching network; determining according to the one or more parametersand according to band information, an impedance mismatch caused by achange in impedance of the antenna; locating, in the look-up table, animpedance mismatch entry that approximates the impedance mismatch,wherein the look-up table comprises a plurality of complex reflectioncoefficients predetermined by measuring a signal at a terminal of thetunable impedance matching network for a plurality of impedance loads ofthe antenna; and adjusting the tunable reactive element according toselected complex reflection coefficients from among the plurality ofcomplex reflection coefficients to perform impedance matching.
 13. Thecommunication device of claim 12, wherein the reactive element comprisesa variable capacitor.
 14. The communication device of claim 13, whereinthe variable capacitor comprises a voltage-controlled variableferroelectric capacitor.
 15. The communication device of claim 14,further comprising a digital to analog converter, wherein the operationsfurther comprise adjusting the voltage-controlled variable ferroelectriccapacitor utilizing the digital to analog converter.
 16. Thecommunication device of claim 12, wherein the tunable reactive elementcomprises a micro-electro-mechanical systems (MEMS) device.
 17. Thecommunication device of claim 12, wherein the tunable reactive elementcomprises a semiconductor device.
 18. The communication device of claim12, wherein the parameters are obtained via the detector while thetunable impedance matching network is in a predetermined state.
 19. Amethod, comprising: obtaining, by a processor of a communication device,one or more parameters via a detector of the communication device forforward and reverse power associated with a tunable impedance matchingnetwork of the communication device; determining, by the processoraccording to the one or more parameters and according to bandinformation, an impedance mismatch caused by a change in impedance of anantenna of the communication device; locating, by the processor in alookup table, an impedance mismatch entry that approximates theimpedance mismatch, wherein the lookup table comprises predeterminedtuning information that is measured for signals at a terminal of thetunable impedance matching network for a plurality of impedance loads ofthe antenna; and adjusting, by the processor, a tunable reactive elementof the tunable impedance matching network according to selected tuninginformation from among the predetermined tuning information to performimpedance matching.
 20. The method of claim 19, wherein thepredetermined tuning information comprises a plurality of complexreflection coefficients.